That's because the number is associated with your apple ID (which ywuse in your iPad and other Apple devices). When you switch simcards, the new phone number will be associated with your ID. Usually you receive a popup messsage telling you that imessage/FT activation (with your new number) will happen (or something similar).
Although sometimes there's been some sort of error that peoples iMessage/FT gets stuck with old phone number, dven thiugh they have a new simcard with different number.
Right now my iPhone's iMessage/FT is set up with my ID and the number from this country, when I go back, and swap simcards, it will change the number and so on.
The best way for you is to try though.